Low resistance barrier layer for isolating, adhering, and passivating copper metal in semiconductor fabrication

ABSTRACT

Cubic or metastable cubic refractory metal carbides act as barrier layers to isolate, adhere, and passivate copper in semiconductor fabrication. One or more barrier layers of the metal carbide are deposited in conjunction with copper metallizations to form a multilayer characterized by a cubic crystal structure with a strong (100) texture. Suitable barrier layer materials include refractory transition metal carbides such as vanadium carbide (VC), niobium carbide (NbC), tantalum carbide (TaC), chromium carbide (Cr 3 C 2 ), tungsten carbide (WC), and molybdenum carbide (MoC).

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the Regents of the University of California for the operationof Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor device fabrication andmore particularly, to low resistance barrier layers for reliablyisolating, adhering, and passivating copper metal and achieving theoptimum cubic crystalline texture in the copper metal.

2. Description of Related Art

Advances in materials technology are playing an important role inimproving semiconductor device performance and the reduction of powerconsumption in solid state electronic components and systems.Conventional semiconductor technologies, e.g., 0.50 μm CMOS, typicallyuse multilevel aluminum alloy metallizations andchemical-vapor-deposited (CVD) tungsten plugs. The tungsten plug processis being replaced by the use of aluminum plugs in the deviceinterconnect contacts and via holes.

Aluminum plugs reduce process complexity, increase manufacturing yield,and decrease interconnect resistance. Although the overall interconnectresistance is being reduced, the power losses in on-chip interconnectstructures remains significant in integrated circuits, particularly asdevice density increases. To reduce such power losses further, themetallization structures need to be made with materials that haveresistivities lower than aluminum and other related new materialstechnologies such as low-k interlevel dielectrics.

Because of its lower bulk and skin resistance, an effective replacementfor aluminum is copper that is isolated from the electrically activedevices by a reliable barrier material. Methods to deposit coppermetallizations include electroless deposition, metal organic chemicalvapor deposition (MOCVD), electroplating, and collimated physical vapordeposition (c-PVD). Regardless of the way copper is deposited, the metalmust be isolated or encapsulated so that it will not diffuse into thesurrounding areas of the device and thus degrade performance.

Copper appears to be the best of the available choices from the list ofknown low resistivity metals, e.g., silver, aluminum, gold, copper, andtungsten. Copper offers many advantages: low resistivity, ease ofdeposition, high thermal conductivity, a lower temperature coefficientof resistance than aluminum and tungsten, a lower coefficient of thermalexpansion than aluminum, the highest melting point except for tungsten,and the lowest adiabatic temperature rise due to Joule heating. Copperis also expected to offer lower electromigration (by several orders ofmagnitude) in poly or single crystalline materials. The copper texturealso enhances performance, yielding lower stresses and other beneficialproperties.

The shrinking feature sizes of ULSI circuits places severe requirementson interconnect metallization technologies, particularly where severetopography exists, such as in submicron diameter contact windows andvias. Since sub-0.25 μm feature size integrated circuits will beperformance limited by the resistance in the metal interconnects, coppermetallization is better than aluminum because of copper's lowerresistivity and higher resistance to electromigration.

Given these advantages, semiconductor manufacturers are expendingsignificant efforts to incorporate copper into upper-levelmetallizations. The use of copper, however, also requires theincorporation of diffusion barriers, adhesion promoters, and passivationlayers. Diffusion barriers that are thermally stable, chemically stable,and electrically conductive are needed to isolate copper due to its highatomic mobility. Copper diffuses rapidly in silicon and dielectrics,which strongly degrades semiconductor device performance, and thusmaterials must be identified that block this diffusion.

Adhesion promoters are needed since copper does not “wet” or bond wellto silicon dioxide and other dielectric surfaces, especially whensubjected to thermal cycling. Passivation layers are required to preventenvironmental degradation of the etched or chemically/mechanicallypolished copper surfaces. Substantial efforts have been made to identifybarrier layer materials that meet all major requirements, e.g.,diffusion barrier, adhesion promoter, passivation barrier, and lowelectrical resistivity for a low contact/via resistance.

Tantalum, TiN, TiCON, and TiOS have been used for barrier layermaterials. Other conventional barrier technologies include amorphousrefractory alloys. These diffusion barrier materials have been shown tobe effective at very high temperatures. While all of these materials caninhibit the movement of copper, none of them optimize the microstructureof the copper as it is deposited. Thus, the potential for significantenhancements in the performance of the metallization are lost. Barriermaterials that produce copper with a very uniform microstructure thatincludes a strong (100) cubic texture and a large grain size aredesirable.

It is the object of the present invention to address the problemsinherent in the conventional barrier systems and provide a lowresistance barrier material that effectively isolates, adheres to, andpassivates copper metal for semiconductor fabrication.

SUMMARY OF THE INVENTION

The present invention is a low resistivity refractory metal carbidebarrier system that reliably isolates, adheres, and passivates coppersurfaces in semiconductor fabrication. This copper metallization barrierlayer controls and optimizes the texture and grain size (i.e.,microstructure) of copper metallizations and thereby maximizes theperformance and reliability of the metallization and overallsemiconductor device. Suitable metal carbide barrier layer materialsinclude carbides of transition metals, such as chromium carbide (Cr₃C₂),vanadium carbide (VC), niobium carbide (NbC), tantalum carbide (TaC),tungsten carbide (WC), and molybdenum carbide (MoC). These materialshave either a cubic (NaCl) structure at equilibrium (e.g., VC, NbC, TaC)or a metastable cubic structure (e.g., Cr₃C₂, MoC, WC) that is formedunder non-equilibrium conditions.

These metal carbides are insoluble or have limited solubility in copperin the solid and liquid states (depending on the specific carbide) andare effective diffusion barriers, i.e., these carbides can block copperdiffusion and isolate copper metallizations from the rest of anintegrated circuit device. Furthermore, these metal carbides are wettedby copper, which is critical to providing excellent adhesion betweencopper and materials such as silicon or conventional dielectrics.

One embodiment of the present invention is a multilayer film of metalcarbide barrier layers and copper layers, where the metal carbide layersafford microstructural control of the copper layers. For example, one ormore barrier layers of chrome carbide (Cr₃C₂) are deposited on thesubstrate (e.g., silicon, silicon dioxide) in conjunction with coppermetallizations. The thickness of the carbide layers may be as thin as200 Å or less. The final, terminating layer of copper of the Cu/Cr₃C₂multilayer may be, and typically is, thicker than the underlying copperlayers. The copper and the carbide barrier materials can be deposited bya variety of processes, such as MOCVD, electroless deposition,collimated physical vapor deposition (c-PVD), magnetron sputtering, andelectroplating.

The present invention provides a robust, production-worthy, integrateddeposition technology for low power, high performance, high reliabilitycopper metallizations with critical dimensions of 0.25 μm and less. Thecubic barrier layer or layers enhance and maintain the (100)crystallographic texture and orientation of the copper metallizations,as well as increase strength, passivation, and thermal stability. Thebarrier layer also isolates the copper from contamination associatedwith subsequent processing. Other objects and advantages of the presentinvention will become apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is incorporated into and forms part ofthis disclosure, illustrates an embodiment of the invention and togetherwith the description, serves to explain the principles of the invention.

FIG. 1A shows schematically a metal carbide layer at the substrateacting as a barrier layer for copper metallizations.

FIG. 1B shows schematically a copper/metal carbide multilayer in whichthe carbide at the substrate acts as a barrier layer for coppermetallizations.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a barrier layer system for coppermetallizations that comprises a refractory metal carbide. The metalcarbide has either a cubic (NaCl) structure at equilibrium or ametastable cubic structure. Suitable metal carbides include transitionmetal carbides such as chromium carbide (Cr₃C₂), vanadium carbide (VC),niobium carbide (NbC), tantalum carbide (TaC), tungsten carbide (WC),and molybdenum carbide (MoC). The barrier layer system may comprise asingle metal carbide layer, or the barrier layers may be depositedalternately with copper layers to form a multilayer. The carbide andcopper may be deposited using conventional processes.

The cubic metal carbide layer promotes a face centered cubic (100)texture and orientation in the growth of the copper during itsdeposition. The carbide barrier layers, particularly in a carbide-coppermultilayer, maintain and enhance the (100) crystallographic texture ofthe copper, thereby increasing the strength, thermal stability, andpassivation of the copper metallizations. Furthermore, the barrierlayers isolate the majority of the copper from contamination that canoccur during subsequent processing steps. The metal carbides areimmiscible or have limited solubility (depending on the carbide) incopper in both the solid and liquid states and are effective diffusionbarriers. Copper wets (or bonds to) these carbides, which providesexcellent adhesion between copper and silicon or standard dielectricmaterials.

The combination of copper and a suitable cubic carbide (e.g., Cr₃C₂, VC,TaC, WC, MoC, NbC) is advantageous because of copper's low resistivity,the ability of the metal carbide to wet copper, and the insolubility ofcopper in the carbides, so no mixing or diffusion occurs between thecopper and carbide layers. The metal carbides are stable in thin layersnext to copper and have low resistivity. Thus, cubic metal carbidesserve as good diffusion barrier materials for copper interconnectstructures on integrated circuits.

In addition, these cubic metal carbides are effective barrier layermaterials for copper metallizations because they produce very strongcubic textured structure (epitaxy) in the copper that is deposited onthe barrier layer. A cubic or textured microstructure yields a bettercopper metallization because smaller thermal stresses are generated astemperature varies. In particular, cubic or (100) textured copper onsilicon has a smaller value of Δ (thermal stress)/Δ(temperature) than(111) aluminum on silicon. This lower stress has a significant impact onthe amplitude of the thermal stresses that arise during use and hightemperature processing steps, which enhances the stability of themicrostructure and the performance and reliability of the coppermetallizations.

FIGS. 1A and 1B show embodiments (not to scale) of barrier layers forcopper metallizations. FIG. 1A shows a simple system, where a singlelayer 10 of a cubic textured metal carbide is deposited on a substrate12. A layer 14 of copper, also having cubic texture, is deposited on thebarrier layer 10. FIG. 1B shows a more complex system, where a Cu/metalcarbide multilayer film 16 is deposited on a silicon substrate 18. Oneexample of a multilayer film 16 is approximately 75 μm in totalthickness, where the individual copper layers 20 are 270 Å thick and thecarbide layers 22 are 16 Å thick. Cross-sectional TEM micrographs of themultilayers show that the films grow with a large columnar grainboundaries 24 having in-plane copper grain sizes (˜0.5 μm) that exceedthe copper layer thickness by a factor of 20. The topmost copper layer26 may be the same thickness as (or thinner than) the underlying copperlayers 20, but typically the top layer 26 is a thicker metallizationline, as illustrated in FIG. 1B.

With very sharp cubic texture, (100) planes of each grain are not onlyaligned parallel to the deposition plane, but their orientation withinthe plane of deposition is also partially aligned from grain to grain.Such crystallographic texturing and microstructure reduce the thermalstress effects in copper lines, e.g., along the length of theinterconnect lines. Such stress reduction minimizes the chance of voidformation and sidewall failure during operation, and enhances thestability of the as-deposited microstructure during subsequentprocessing steps. The reduction in thermal stresses, the increasedstability of the microstructure, and the large grain size provided bythis invention will result in significant improvements in theperformance and the reliability of copper metallizations.

The ability of copper to grow abnormally wide grains on the carbide isattributed, in part, to copper's ability to wet the carbide. Thischaracteristic can also be seen in TEM micrographs, which show smooth,semi-coherent interfaces between the two sputter deposited materials.The layers are nearly atomically flat and uniform. There may be slightlattice mismatch between the copper and the metal carbide; copper andCr₃C₂, for example, have a mismatch of approximately 4%, creatingperiodic edge dislocations at the Cu/Cr₃C₂ interface. TEM micrographsshow the texturing of the materials: Cr₃C₂ is crystalline in its firstlayering on copper, and copper adopts a cube texture within its firstfew layers.

Multilayer films of copper/metal carbide, such as Cu/Cr₃C₂, have beencharacterized using x-ray diffraction (XRD), transmission electronmicroscopy (TEM), differential scanning calorimetry (DSC), andresistivity measurements. The multilayer films are strong and highlyconductive. Various Cu/Cr₃C₂ multilayer films having differentthicknesses were deposited by magnetron sputtering from targets ofcopper and Cr₃C₂ at powers ranging from 100-500 W. The distance fromsource to substrate varied from 3.9 inches to 5.3 inches.

Calorimetry measurements have shown that Cr₃C₂ layers as thin as 16 Åare stable in copper up to 650° C., and thus thicker barrier layersshould be stable to even higher temperatures. When a multilayer filmsuch as shown in FIG. 1 is heated to 725° C. (at 100° C./min or 20°C./min), the 16 Å Cr₃C₂ layers decompose to form small, oval shapedparticles that are uniformly distributed within the original layers inthe film. This decomposition of the layers reduces the total interfacialenergy of the system. After the particles form, they coarsen with timebut no additional exothermic heat is observed on a second scan of thefilm to 725° C. Thus, the cubic texture of the copper remains stable,i.e., the copper layers have very strong cubic texture both before andafter heating, even though the Cr₃C₂ layers coarsen into particles. Inall TEM analyses, no interface reaction or interdiffusion was observed.

The chemical wetting of the metal carbide by copper and the latticematching between copper and the carbide produce strong adhesion and veryuniform interfaces between the two materials. Both factors are importantin generating the very strong cubic texture (both normal to and in theplane of the layers) that is observed. Because of the intensity of thetexturing, this layered composite can be described as a mosaic singlecrystal.

To investigate the initiation of texturing, a series of depositions wereperformed to see what combination of copper and barrier layerthicknesses produced the most cubic texture in the multilayer films. Thedegree of (100) cubic texture is estimated by comparing the intensitiesof the (111) and (100) peaks. The ratio I₍₁₁₁₎/I₍₁₀₀₎ is cited forsymmetric XRD scans. For a random copper film, I₍₁₁₁₎/I₍₁₀₀₎ is 2/1.Values below this ratio suggest cubic (100) texture, and values abovethis ratio suggest (111) texture.

For example, a multilayer of over 2100 layers of Cu/Cr₃C₂ (270 Å/16 Å)produces a low ratio of (111) to (100) peak intensity equal to 0.01, avery strong cubic texture. Multilayers of copper and Cr₃C₂ with a totalthickness of about 1.0 μm (10,000 Å) were also tested. Multilayershaving different combinations of numbers of layers and layer thicknesseswere made, which showed (111)/(100) intensity ratios ranging from 1/12(cubic) to 10/1 (111) to 1/1.5 (weak cubic). Multilayers were formed inwhich the carbide layers clearly cube the copper texture. The essentialfeature of the carbide layer(s) is that the layer(s) be thick enough(i.e., greater than 10 Å, and typically less than 500 Å, or even 200 Å)so as to form crystals that promote cubic microstructure in the copper.The thickness of the copper layers is typically less than 1000 Å. Inmultilayer films, the number of layers is also a consideration; too fewlayers may generate a weakly cubic texture.

To deposit 1.0 μm copper with strong cubic texture, several depositionswere performed using different combinations of multilayer undercoats andcopper layer thicknesses. The copper films were fabricated both bydepositing many thin layers of copper and by using one 10,000 Å layer ofcopper with different underlayers. Generally, the texture of the singlelayer copper film followed the texture of the underlayer, and oftenincreased the texture of the substrate. Thus, cubic textured copper isgenerated by a cubic underlayer, and not generated from a single carbideunderlayer that is amorphous.

The degree of texture obtained in a copper layer or Cu/Cr₃C₂ multilayervaries with thicknesses and numbers of individual layers of copper andmetal carbide, and may vary with other processing conditions such asdeposition rate and deposition environment. A terminating layer ofcopper on a Cu/carbide multilayer will generally maintain the texture ofthe underlying multilayer.

The effectiveness of Cr₃C₂ barrier layers on silicon substrates wastested by annealing (e.g., 20 minutes at 500° C). Two definitive resultswere obtained. First, when copper is in direct contact with silicon, allcopper is lost into the wafer. No copper peaks could be obtained afterannealing, even for films that initially had a full 1.0 μm of copper asdeposited. Second, when Cr₃C₂ is in direct contact with the silicon, thecopper layers above it do not disappear by diffusing into the silicon.Even 50 Å of Cr₃C₂ is enough of a barrier layer to prevent large scalediffusion of copper into silicon.

Electrical resistivities of the Cu/Cr₃C₂ composite structure weremeasured using a four point testing rig. The electrical resistivity ofthe multilayer is 2.95 μΩ-cm as deposited, 2.67 μΩ-cm after heating to400° C., and 2.29 μΩ-cm after heating to 725° C. The Cr₃C₂ itself has aresistivity of 130-140 μΩ-cm. These results suggest that the density ofcrystalline defects is being reduced, which in turn lowers theresistivity of the film.

Thus, metal carbides having a cubic crystal structure can be used as aneffective barrier layer for copper metallizations on silicon ordielectric substrates. The foregoing description of preferredembodiments of the invention is presented for purposes of illustrationand description and is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. The scope of theinvention is to be defined by the following claims.

What is claimed is:
 1. A semiconductor device having coppermetallizations isolated from a substrate consisting essentially of: asubstrate; and a barrier layer system, deposited on the substrate,having a cubic or metastable cubic crystal texture and comprising aplurality of alternating layers of a refractory metal carbide andcopper, the layers of refractory metal carbide having a thickness thatpromotes cubic crystal texture in the copper layers, and a top copperlayer deposited on the barrier layer system, the thickness of the topcopper layer being greater than an underlying copper layer.
 2. Thesemiconductor device as recited in claim 1, wherein the refractory metalcarbide is selected from vanadium carbide (VC), niobium carbide (NbC),tantalum carbide (TaC), chromium carbide (Cr₃C₂), tungsten carbide (WC),and molybdenum carbide (MoC).
 3. The semiconductor device as recited inclaim 1, wherein the barrier layer system has a thickness less thanabout one micrometer.
 4. The semiconductor device as recited in claim 1,further comprising a copper layer deposited on the barrier layer system.5. The semiconductor device as recited in claim 1, wherein each copperlayer has a cubic crystal texture.
 6. The semiconductor device asrecited in claim 1, wherein each layer of the refractory metal carbidehas a thickness less than about 500 Å.
 7. The semiconductor device asrecited in claim 1, wherein each layer of the refractory metal carbidehas a thickness of less than about 200 Å.
 8. The semiconductor device asrecited in claim 1, wherein each layer of the copper has a thickness ofless than about 1000 Å.
 9. The semiconductor device as recited in claim1, wherein the layers of the refractory metal carbide have a thicknessless than the thickness of the layers of copper.
 10. The semiconductordevice of claim 1, wherein the layers of refractory metal carbide areselected from the group consisting of materials having a cubic structureat equilibrium and metastable cubic structure formed undernon-equilibrium conditions.
 11. The semiconductor device of claim 1,wherein the layers of refractory metal carbide are selected from thegroup consisting of chromium carbide, vanadium carbide, niobium carbide,tantalum carbide, tungsten carbide, and molybdenum carbide.